Solid-state lighting arrays are used for a number of lighting applications. For example, solid-state lighting panels including arrays of solid-state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid-state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs), which may include inorganic LEDs, which may include semiconductor layers forming p-n junctions and/or organic LEDs (OLEDs), which may include organic light emission layers.
Solid-state lighting arrays are used for a number of lighting applications. For example, solid-state lighting panels including arrays of solid-state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. Solid-state lighting devices are also used in lighting fixtures, such as incandescent bulb replacement applications, task lighting, recessed light fixtures and the like. For example, Cree, Inc. produces a variety of recessed downlights, such as the LR-6 and CR-6, which use LEDs for illumination. Solid-state lighting panels are also commonly used as backlights for small liquid crystal display (LCD) screens, such as LCD display screens used in portable electronic devices, and for larger displays, such as LCD television displays.
A solid-state light emitting device may include, for example, a packaged light emitting device including one or more LEDs. Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid-state light emitting device. Typically, a solid-state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region.
Many control circuits for lighting utilize phase cut dimming. In phase cut dimming, the leading or trailing edge of the line voltage is manipulated to reduce the RMS voltage provided to the light. When used with incandescent lamps, this, reduction in RMS voltage results in a corresponding reduction in current and, therefore, a reduction in power consumption and light output. As the RMS voltage decreases, the light output from the incandescent lamp decreases.
An example of a cycle of a full wave rectified AC signal is provided in FIG. 1A, a cycle of a phase cut (“leading edge”) rectified AC waveform is illustrated in FIG. 1B and a cycle of a reverse phase cut (“trailing edge”) AC waveform is illustrated in FIG. 1C. As seen in FIGS. 1A through 1C, when phase cut dimming is utilized, the duty cycle of the resulting rectified waveform is changed. This change in duty cycle, if sufficiently large, is noticeable as a decrease in light output from an incandescent lamp. The “off” time does not result in flickering of the incandescent lamp because the filament of an incandescent lamp has some thermal inertia and will remain at a sufficient temperature to emit light even during the “off” time when no current flows through the filament.
Recently, solid state lighting systems have been developed that provide light for general illumination. These solid state lighting systems utilize light emitting diodes or other solid state light sources that are coupled to a power supply that receives the AC line voltage and converts that voltage to a voltage and/or current suitable for driving the solid state light emitters. Typical power supplies for light emitting diode light sources include linear current regulated supplies and/or pulse width modulated current and/or voltage regulated supplies.
Many different techniques have been described for driving solid state light sources in many different applications, including, for example, those described in U.S. Pat. No. 3,755,697 to Miller, U.S. Pat. No. 5,345,167 to Hasegawa et al, U.S. Pat. No. 5,736,881 to Ortiz, U.S. Pat. No. 6,150,771 to Perry, U.S. Pat. No. 6,329,760 to Bebenroth, U.S. Pat. No. 6,873,203 to Latham, II et al, U.S. Pat. No. 5,151,679 to Dimmick, U.S. Pat. No. 4,717,868 to Peterson, U.S. Pat. No. 5,175,528 to Choi et al, U.S. Pat. No. 3,787,752 to Delay, U.S. Pat. No. 5,844,377 to Anderson et al, U.S. Pat. No. 6,285,139 to Ghanem, U.S. Pat. No. 6,161,910 to Reisenauer et al, U.S. Pat. No. 4,090,189 to Fisler, U.S. Pat. No. 6,636,003 to Rahm et al, U.S. Pat. No. 7,071,762 to Xu et al, U.S. Pat. No. 6,400,101 to Biebl et al, U.S. Pat. No. 6,586,890 to Min et al, U.S. Pat. No. 6,222,172 to Fossum et al, U.S. Pat. No. 5,912,568 to Kiley, U.S. Pat. No. 6,836,081 to Swanson et al, U.S. Pat. No. 6,987,787 to Mick, U.S. Pat. No. 7,119,498 to Baldwin et al, U.S. Pat. No. 6,747,420 to Barth et al, U.S. Pat. No. 6,808,287 to Lebens et al, U.S. Pat. No. 6,841,947 to Berg Johansen, U.S. Pat. No. 7,202,608 to Robinson et al, U.S. Pat. No. 6,995,518, U.S. Pat. No. 6,724,376, U.S. Pat. No. 7,180,487 to Kamikawa et al, U.S. Pat. No. 6,614,358 to Hutchison et al, U.S. Pat. No. 6,362,578 to Swanson et al, U.S. Pat. No. 5,661,645 to Hochstein, U.S. Pat. No. 6,528,954 to Lys et al, U.S. Pat. No. 6,340,868 to Lys et al, U.S. Pat. No. 7,038,399 to Lys et al, U.S. Pat. No. 6,577,072 to Saito et al, and U.S. Pat. No. 6,388,393 to Illingworth the disclosures of which are hereby incorporated herein by reference.
In the general illumination application of solid state light sources, one desirable characteristic is to be compatible with existing dimming techniques. In particular, dimming that is based on varying the duty cycle of the line voltage may present several challenges in power supply design for solid state lighting. Unlike incandescent lamps, LEDs typically have very rapid response times to changes in current. This rapid response of LEDs may, in combination with conventional dimming circuits, present difficulties in driving LEDs.
The switch or circuit element that controls the power on-off inside a typical phase control dimmer is typically a type of thyristor device commonly known in the art as a TRIAC. TRIACs generally have a first main terminal MT1 a second main terminal MT2 and a gate terminal G and allow bidirectional conduction through the main terminals, allowing AC to pass through. The TRIAC is turned on and conduction is present between the main terminals when there is a trigger current present between gate G and second main terminal MT2. Once triggered, the TRIAC remains on until a zero crossing of the AC power line at which point the device turns off and awaits the next trigger pulse or zero crossing of the AC power line. This characteristic allows phase angle control to be achieved.
A TRIAC will not remain in the on state after triggering without a current larger than the hold current passing through the main terminals. Because of the need to hold a current, TRIACs may have difficulty remaining on when a low current is drawn through the main terminals, such as in the case of LED lighting. Some TRIACs may have a hold current of around 20 milliamps.
LED lighting is generally more energy efficient that incandescent light. A typical incandescent light bulb can easily draw more than 200 mA during conduction. This value largely exceeds the holding current of typical dimmers. Therefore, there is usually no problem in dimming an incandescent bulb. LED lighting generally draws less current, typically ranging from 10 to 150 mA depending on the circuit design. At smaller current levels, once the dimmer conducts, the load current may not satisfy the hold current requirement of the TRIAC in the dimmer, and the dimmer may enter a retriggering state that causes flickering of the LED light.